The present invention relates to a MIS transistor having a polysilicon gate electrode provided on a gate insulating film and, more particularly, to an improvement in the driving force thereof.
Doping of the gate electrode of a MOS transistor with an impurity has been performed conventionally and commonly to reduce the resistance of the gate electrode and thereby improve the driving force of the MOS transistor. In particular, a CMOS device typically has a so-called dual gate structure in which the gate electrode of an n-channel MOS transistor is doped with an n-type impurity (phosphorus or arsenic) and the gate electrode of a p-channel MOS transistor is doped with a p-type impurity (boron).
However, since boron as the p-type impurity is particularly likely to be diffused in an oxide film, a phenomenon of so-called “boron penetration” is observed in which boron passes through the gate oxide film to enter the channel region of a Si substrate, which depletes the gate electrode. This causes the disadvantages of a lower driving force of the p-channel MOS transistor, degraded subthreshold characteristics, and aggravated short-channel effects.
To prevent these disadvantages, there has been used a technique for suppressing downward diffusion of boron by composing the gate insulating film of a silicon oxynitride film. In using the technique, there has commonly adopted a method of forming a silicon oxynitride film directly on a silicon substrate by performing a heat treatment, while allowing a gas mixture of oxygen and NO or an N2O gas to flow over the silicon substrate, or a method of changing a silicon oxide film already formed into a silicon oxynitride film by allowing an N2O gas or NH3 gas to flow over the silicon oxide film and thereby introducing nitrogen into the silicon oxide film.
However, as the gate length of a transistor becomes shorter with the increasing miniaturization and higher integration of a CMOS device currently pursued, a gate insulating film is also reduced in thickness to provide a sufficient driving force despite a reduction in voltage, which has caused the problem that a transistor with a sufficient driving force is not provided even if the gate insulating film is composed of a silicon oxynitride film.
Although the cause thereof has not been determined yet with certainty, the present inventors have experimentally assumed that an insufficient driving force is attributable to a mechanism other than the depletion of a gate electrode and, in particular, to a poor state of nitrogen distribution in the silicon oxynitride film as the gate insulating film.